EP0735354B1 - Mikromechanischer Drucksensor mit erweitertem Messbereich - Google Patents

Mikromechanischer Drucksensor mit erweitertem Messbereich Download PDF

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Publication number
EP0735354B1
EP0735354B1 EP96301794A EP96301794A EP0735354B1 EP 0735354 B1 EP0735354 B1 EP 0735354B1 EP 96301794 A EP96301794 A EP 96301794A EP 96301794 A EP96301794 A EP 96301794A EP 0735354 B1 EP0735354 B1 EP 0735354B1
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Prior art keywords
vibrating
gauge
pressure
stationary member
mass
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English (en)
French (fr)
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EP0735354A1 (de
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Jacob H. Martin
William P. Kelley
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Varian Inc
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Varian Inc
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L21/00—Vacuum gauges
    • G01L21/16—Vacuum gauges by measuring variation of frictional resistance of gases
    • G01L21/22—Vacuum gauges by measuring variation of frictional resistance of gases using resonance effects of a vibrating body; Vacuum gauges of the Klumb type
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
    • G01L9/0001—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means
    • G01L9/0008—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means using vibrations
    • G01L9/0019—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means using vibrations of a semiconductive element

Definitions

  • This invention relates to an improved molecular drag type pressure gauge, and in particular to a molecular drag type guage having an extended pressure range measurement capability.
  • Pressure measurement in vacuum systems is particularly challenging because of the enormous range of pressures that can be realized.
  • Typical vacuum systems have two or more types of gauges, each with its particular range of usefulness. The need to switch between different vacuum gauges is tiresome and produces reading discontinuities where the gauge ranges meet. Gauges with broad pressure ranges are attractive because they can reduce the number of different types of gauges needed to monitor a particular vacuum system.
  • molecular drag gauges make use of the phenomenon that at low pressures the drag forces produced by a gas on an object moving through it are proportional to the pressure of the gas.
  • One type of gauge employing this principle uses a freely swinging fibre or vane as a means of measuring pressures in the range of 0.133 to 0.00133 Pa.
  • the fibre or vane pendulum is started swinging mechanically, for example by mechanically shaking the vacuum container.
  • the damping is due mainly to the gas in the container.
  • the time for the pendulum to damp to one half its original amplitude, or half-life is measured.
  • the relationships between damping and pressure and damping and half-life can be used to determine the pressure as a function of half-life.
  • This method of measuring pressure is quite limited in range. It is also cumbersome and takes on the order of one hour to make a measurement at low pressure.
  • tuning fork made from piezoelectric material as the sensing element.
  • the tuning fork is made to oscillate and its resonance resistance is directly proportional to gas pressure when the pressure is low enough to be in the molecular flow region.
  • the resonance resistance continues to increase with pressure, but at a much reduced rate.
  • the tuning fork oscillator the tuning fork is placed where the pressure is to be measured and caused to oscillate by means of an oscillator circuit. The pressure is determined by measuring the difference between the resonant resistance where the pressure is being measured and the natural resonance resistance of the tuning fork.
  • One of the drawbacks of this device is that its range is limited at the low end when the resistance caused by the gas is of the same order as the natural resonance resistance of the tuning fork.
  • the sensitivity is also limited at the high end by the shift from molecular resistance to the transition between molecular and viscous resistance.
  • a spinning rotor gauge measures the deceleration of a magnetically levitated spinning metal sphere inside a stainless steel chamber that is, in turn, immersed in the gas that is to have its pressure measured.
  • the ball is electromagnetically spun up to a target rotation rate and then allowed to decelerate.
  • the rate of the ball's deceleration is proportional to the number of gas molecules that come in contact with the ball per unit time which is, in turn, proportional to gas pressure.
  • This gauge can measure pressures in the range of 1.33 to 6.6 x 10 -5 Pa. Spinning ball gauges are very accurate, however their use is restricted by their size, high cost and limited range measurement capability.
  • the present invention can utilize a micromachined mass and spring system that is damped in proportion to the pressure of the gas which surrounds it.
  • the mass motion is electrostatically excited and sensed, with the measured pressure range being extended by using a spring system comprised of a material having a very low internal damping, such as silicon, preferably single crystal silicon.
  • the measurement range is extended at the low pressure end by the use of what is referred to herein as "squeeze damping", which is a special case of molecular damping, to increase the amount of vibrational damping to measurable levels.
  • the measurement range is likewise extended to higher pressures by using micromechanical technology to minimize the distance between the vibrating member and a stationary member, and thereby extend the damping measurement within the molecular flow regime, or molecular drag, to higher pressures.
  • the transition region between this molecular damping and higher pressure viscous damping is called slip damping and extends over approximately 2 orders of magnitude of pressure.
  • the drag in the slip damping regime is not as pressure dependent as molecular drag, but it is still useful as a pressure indicator. Viscous damping is not pressure dependent.
  • the practical range for measuring molecular drag, and therefore pressure, for a single sensor having a spring system comprised of a material with an intrinsic Q of well over 100,000 is between about 4 and 5 orders of magnitude.
  • This difficulty is overcome with the pressure gauge of the present invention by either using more than one sensor, or by designing a single sensor to operate in two vibrational modes, one mode to make use of squeeze damping and the other to operate in the molecular drag and slip damping regimes.
  • An electronic system is provided that measures the drive voltage necessary to maintain a predetermined oscillation amplitude of the vibrating member. This voltage, which is proportional to the damping of the vibrating member and thus pressure, is converted by the system to a pressure reading.
  • the gauge of the present invention does not rely on the vibrating member being stressed, and the frequency of the gauge remains constant with changing pressure. By measuring the drive voltage required to maintain the predetermined oscillation amplitude at a constant frequency, a measure of the gas pressure is obtained.
  • the voltage is measured to maintain a fixed amplitude, since it is desired to operate a gauge of the claimed invention in the molecular drag and squeeze drag regimes. It is achieved by arranging the close proximity of the stationary member to the vibrating member. In these drag regimes and at a constant drive voltage, the amplitude would change by an order of magnitude for every order of magnitude change in pressure, which gives good resolution. However, large amplitudes will cause the mechanical oscillator to go non-linear and possibly hit its stops or go into clampdown. That is the reason why fixed amplitude operation is preferred for the claimed device.
  • Fig. 1 is a plot showing the relationship of both the quality factor Q and the drive voltage for constant amplitude as a function of pressure for sensors of various designs.
  • Fig. 2 is a plan view of a cantilever beam pressure sensor, which represents the simplest vibrating spring and mass system of this invention.
  • Fig. 2b is an elevational view of the cantilever beam pressure sensor shown in Fig. 2a.
  • Fig. 3a is a plan view of a seesaw pressure sensor cofiguration of the present invention that can be driven by two drive voltages of equal amplitude, but 180 degrees out of phase.
  • Fig. 3b is an elevational view of the sensor in Fig. 3a.
  • Fig. 4a is a plan view of a sensor with a large-area perforated mass supported by 4 springs.
  • Fig. 4b is a elevational view of the sensor in Fig. 4a.
  • Fig. 5a is a plan view of an alternative embodiment of the sensor of the present invention where the mass can be driven in two modes, one mode parallel to a support member, for measuring pressures in the intermediate to high range, and one mode perpendicular to the support member, for measuring pressures in the intermediate to low range.
  • Fig. 5b is an elevational view of the sensor shown in Fig. 5a.
  • Fig. 6 is a schematic for the pressure sensor interface electronics of the present invention.
  • Fig. 7 is a schematic for the pressure sensor readout and drive electronics of the present invention.
  • the design and operation of the present invention is based on the pressure dependent gas drag on a moving body, or damping effect, when the gas is in the molecular drag regime, refered to herein as molecular damping.
  • the invention provides the means for extending the overall range of pressure measurement of this type of gauge by extending the molecular drag measurement range to pressures higher than those attainable with conventional drag dependent pressure sensors, and by providing a greatly increased drag or damping effect at lower pressures.
  • the drag force produced on a moving body in a gas is proportional to the gas pressure when the pressure is within in the molecular flow regime.
  • One way of defining the molecular flow regime is that it occurs when the gas molecules are more likely to hit the walls of the chamber in which they are contained than they are to hit each other. In terms of the gas mean free path, if it is larger than the chamber dimensions, then the gas is in the molecular regime. It is recognized by those skilled in the art that the molecular flow regime can be extended to higher pressures by making the containment vessel smaller. However, the gas does not have to be in a closed container at all.
  • the sensor characterized in plot 11 in Fig. 1 continues to indicate a measured change in Q with pressure, but at a slower rate. This is the transition region,starting at point A, between molecular drag and viscous drag regimes and is called slip drag. At still higher pressures the measured Q will no longer change with pressure.This is the viscous drag regime.
  • a practical range for the measurement of Q for a pressure sensor of the present invention is about 5 to about 100,000. At higher Q values the signal levels become very small and therefore less accurate; or if the time decay method is used, the measurement times become very long.
  • the intrinsic Q of a single crystal silicon spring system which is preferred for use in the present invention as an oscillating means, can be neglected below 100,000,but starts to become an important factor at higher Q levels. For Q below 5 pressure measurement errors become significant because the effective Q differs from the measured Q, and the linear dependency between Q and drag breaks down.
  • the practical limitations imposed on the Q range limits the pressure range that can be measured using a single mode sensor.
  • Fig. 1 shows curves for two such sensors, one designed for operation at higher pressures, on the order of about 10 -3 Torr (0,13 Pa) to about 10 Torr (1,3 kPa) ( plot 12), and one designed to operate at lower pressures, on the order of about 10 -2 (1,3 Pa) to less than about 10 -6 Torr (0,13 mPa) (plot 13).
  • An alternative, and preferred embodiment for cost and space utilization purposes utilizes a single sensor, where the mass vibrates in two modes, one parallel to the supporting substrate member and one perpendicular to it.
  • the parallel motion can be optimized to measure in the upper half of the pressure range and the perpendicular mode can be optimized to operate in the lower half of the range.
  • a cantilever beam 21 is supported by electrically insulative substrate 22 such as pyrex by anchor point 23.
  • the cantilever beam acts as both mass and spring in this system, as well as the first drive electrode. It is made of doped silicon, which is electrically conductive, and is provided an electrical connection to the outside world through anchor point 23 and the conductive path provided by metallization 24.
  • the second electrode 25 of the electrostatic drive is supported by substrate 22 and is connected to the outside world via metallization 26.
  • the driving force for the oscillator is periodic and will be considered as sinusoidal.
  • the damping force is provided by the gas surrounding the oscillating element.
  • the equation of motion for this damped system can be written in the form: where x is the displacement of the mass from its rest position. The displacement of the mass is resisted by the spring force, which is k times the displacement, and the drag force, K times the velocity.
  • the oscillator is driven by the force, Csin ⁇ t.
  • the electrostatic drive is achieved by applying a sinusoidal voltage between the conductive vibrating mass element, which for the system depicted in Fig. 2a is the cantilever beam 21, and an opposing electrode on the supporting member 22, which in Fig. 2a is the substrate electrode 25.
  • the force on the mass is caused by the applied voltage acting on the charge that accumulates on the mass, which is acting as one plate of a capacitor.
  • V 0 V term dominates as long as V 0 >>V.
  • V 0 bias voltage
  • V 0 + V sin ⁇ t were used to drive a support electrode under one side of a torsion-spring mounted seesaw mass
  • V 0 - V sin ⁇ t were used to drive a support electrode on other side
  • the effects of the ( V 2 /2) cos 2 ⁇ t terms and the voltage bias terms V 0 will be cancelled.
  • FIG. 3b show a seesaw mass 31 supported by torsion springs 32a and 32b which are in turn attached to substrate 33 by anchor points 34a and 34b.
  • Mass 31 also acts as one of the drive electrodes and is electrically connected to the outside world through metallization 35.
  • Two substrate drive electrodes, 36a and 36b, are connected to the outside world via substrate metallizations 37a and 37b. Cancellation of the effect of the 2 ⁇ term can alternatively be achieved by suspending two identical planar masses symmetrically by a common spring system, which is driven with the same complementary drive voltages as described herein for the seesaw configuration.
  • the cancellation of the unwanted AC term can also be realized by using the same combination of drive voltages previously described on two comb drives, positioned on opposite sides of an oscillating mass, to drive the mass parallel to the support member. Such an embodiment is shown in Fig. 5.
  • ⁇ is set to ⁇ k/m.
  • damping regimes that are useful for the pressure gauge of this invention.
  • molecular damping at intermediate pressures squeeze damping (an enhanced type of molecular damping) at low pressures
  • slip damping at higher pressures The pressure ranges for these damping regimes will vary depending on the specific operational parameters required, which in turn will dictate the sensor design specifications.
  • the low pressure sensor would utilize a large mass plate, on the order of 20 mils across its smallest area dimention, spaced 2 microns or less from the support member.
  • squeeze damping an enhanced type of molecular damping
  • the high pressure sensor could use a mass of similar dimensions, also spaced 2 microns or less from the support substrate, however the direction of oscillation would be driven parallel to the support plane. If operational restrictions require the high pressure sensor to oscillate in a perpendicular direction, a ventillated mass plate would be used to prevent the squeeze damping effect.
  • the webs between the holes in the mass plate should be no wider than the mass to substrate distance for a maximum reduction in the squeeze damping phenomenon.
  • a pressure gauge having sensors of the type described herein will operate in the squeeze damping regime, for the low pressure sensor, between about 10 -6 Torr (0,13 mPa) and about 10 -2 Torr (1,3 Pa).
  • operation in the non-squeeze/ molecular damping regime is between about 10 -3 Torr (0,13 Pa) and about 10 Torr (1,3 kPa), and in the slip damping regime between about 10 Torr (1,3 kPa) and about 1000 Torr (133 kPa).
  • these ranges can be moved up or down by providing: changes in the spacing between the vibrating member and the support member; variations in the vibrating member area; variations in the direction of oscillation of the vibrating member; and by changing the resonant frequency of the spring and mass system.
  • the shape of the vibrating member can also affect the system's range of operation.
  • a ventillated round plate mass driven in perpendicular oscillations wherein the shape is such that the minimum cross dimention is maximized, will have higher damping in the squeeze mode and will extend the measurement range to higher pressures than a plate of reduced area, as a result of the incorporation of holes spaced throughout the plate and the selection in the plate mass shape. It is an object of this analysis to show how these design parameters can be manipulated to make individual pressure sensors, each capable of measuring a different pressure range, for use in combination within a single pressure gauge, to provide an overall extended measurement range capability.
  • the damping factor for molecular damping is given the subscript m and is K m ⁇ P M T A where P is the pressure, M is the mass of the gas molecule, T is the absolute temperature and A is the area on which the damping force acts.
  • P is the pressure
  • M is the mass of the gas molecule
  • T is the absolute temperature
  • A is the area on which the damping force acts.
  • the damping effect can be changed for a plate mass that is oscillating in a direction perpendicular to its planar axis by placing a second plate, which can also act as the support structure, parallel to the oscillating plate and in close proximity to it.
  • gas is alternately forced out from between the plates and then sucked back in by the motion of the oscillating plate.
  • This pumping action is called "squeeze damping".
  • the squeeze damping can be reduced by perforating the plate with holes, by increasing the spacing between the plate and the support structure, and by reducing the plate size.
  • the squeeze damping will revert to conventional molecular damping if the motion of the oscillating plate is changed to a vibratory motion which is parallel to the plane of the plate mass.
  • the damping factor for squeeze damping is given the subscript sq and is K sq ⁇ P M T A ( C s W 3 d 3 ) where C s is a constant that depends on the shape of the mass, d is the distance between the plate mass and the support member, and W is a mass dimension, such as the edge length for a square mass, or a diameter for a round mass.
  • the bracketed quantity can be driven to unity, which takes the sensor out of the squeeze damping regime, by decreasing W or increasing d.
  • FIG. 4a and Fig. 4b an embodiment of a single sensor designed in accordance with the present invention is shown, having large area mass 41 supported by 4 springs 42a, 42b, 42c, and 42d.
  • a large area mass for present micromechanical technology is on the order of 20 mils ( approximately 500 microns) across for a mass-to-fixed-member spacing of about 1 or 2 microns. This limit results from a tendancy for the plates to curl when larger mass areas are manufactured, which increases the likelihood of contact between the mass and substrate during operation.
  • the springs are anchored to substrate 47 at anchor points 43a, 43b, 43c, and 43d.
  • Mass 41 serves as one of the drive electrodes and is electrically connected to the outside world via spring 42d, anchor point 42d, and substrate metallization 44.
  • Substrate electrode 45 is connected to the outside world via substrate metallization 46.
  • Holes 48 in the mass may be incorporated to greatly reduce the squeeze damping effect depending on the operational requirements of the sensor. If squeeze damping is to be largely eliminated then the webs between the holes must be smaller than the mass-to- substrate distance. On the other hand fabrication methods that do not require holes must be employed to achieve the most effective squeeze damping.
  • the damping factor for slip damping is less dependant on pressure than molecular damping, and occurs over approximately 2 decades of pressure that transitions between molecular damping and viscous damping, where viscous damping has no dependance on pressure.
  • Slip damping on sensors of the type described herein has been found to have the following dependance on pressure: K sl ⁇ P n M T A where n is on the order of 1/4.
  • the senor must be designed to have its mass move parallel to the fixed support member, or if the mass is designed to move perpendicular to the fixed member, its width must either be on the order of its spacing from the fixed member, or if the mass width is larger than the fixed member spacing, the mass must contain many closely spaced holes to prevent the squeeze damping effect.
  • equation (3) is written in terms of equation (11) for C and equation (13) for K sq X SQ ⁇ d ⁇ 0 V 0 W 3 C s M T k m ( V P )
  • Equation (16) shows that the amplitude can be kept small by making the distance d between the mass M and fixed support member is small ,and by maximizing the plate mass edge dimension W.
  • the bias voltage will always be limited since its effect is to draw the vibrating member closer to the stationary member. If the bias voltage is too high, the electrostatic force will overcome the spring force and the vibrating member will "snap down" to the stationary member.
  • maintaining the resonant frequency, ⁇ k/m, as high as allowable by the system readout electronics will also keep the vibrational amplitude within bounds at low pressures.
  • Fig. 5a and Fig. 5b show another alternative embodiment of the present invention, wherein the sensor is designed to operate in two vibration modes, one mode for measuring pressures at the low end of the pressure range, and another mode for measuring at the higher end of the pressure range.
  • the high end pressure sensor can also be used to measure pressures which fall within the slip regime, and thereby extend the overall measurement range to still higher pressures, albeit with lower accuracy.
  • Mass 51 is driven parallel to fixed support substrate 52 by comb electrodes 53a and 53b for higher pressure measurements. Mass 51 is driven to oscillate in a direction perpendicular to the substrate by substrate electrode 54 for lower pressure measurements.
  • the comb drive would use complementary AC drive voltages on the opposing comb drives, as previously described, but it is also possible to drive the mass with a single AC voltage on 1 comb drive.
  • the squeeze mode is driven with a single AC voltage, however, those skilled in the art will recognize that it is possible to build a two mass system, each mass with dual comb drives, where both vibrational modes can be driven with a complementary AC drive.
  • the frequencies of the two modes can designed to be different by making springs 55a,55b,55c,and 55d rectangular in cross section, and the frequencies can be adjusted, either up or down, by varying the cross section and length of the springs.
  • the springs can alternatively be folded, so that anchor points 56a, 56b, 56c, and 56d are closer together, thereby reducing the differential thermal expansion effects between the substrate and spring mass system. This is useful when the sensor is to be used over a wide temperature range, where temperature control of the sensor might otherwise be called for.
  • the spring system of the present invention should preferrably be constructed of a material that is easily micromachined, has very low internal damping, and has essentially an infinate life at the displacements contemplated. It is also advantageous if, in addition to the above properties, the spring material is electrically conductive.
  • the preferred material for making the spring and mass system is silicon, and in particular single crystal silicon that has been doped with a material to make it conductive, such as boron .
  • the substrate or fixed support member, must be an electrical insulator, or have an insulative surface, and must have a thermal coefficient of expansion close to that of the mass and spring material. Pyrex glass and passivated silicon are preferred materials, however other glass materials and ceramics could also be used for the substrate.
  • the drive electrodes are connected to outside power sources via substrate metallizations 57a, 57b, 58 and 59.
  • Device bias voltages up to 5 volts, and drive voltages in the range of 50 microivolts to 5 volts, are sufficient to drive the sensor system of the present invention.
  • Drive voltages below 50 microvolts would cause noise and distortion problems, and drive voltages much higher than 5 volts would require higher voltage electronic components.
  • the bias voltage, V 0 could also be varied to extend the drive force range beyond the 5 orders of magnitude suggested by the drive voltage range.
  • the practical Q range for the sensor is in the 5 to 100,000 range so a larger drive voltage range is not required.
  • the pressure sensor described herein has the following phase response characteristics. For frequencies below resonance the output amplitude is constant for a constant amplitude drive voltage, and the sensor output is in phase with the drive signal. At resonance, the sensor output is Q times the output amplitude below resonance and the output signal lags the drive voltage by 90 degrees. For frequencies just above resonance, the sensor output signal decreases at greater than -40dB/decade, and for frequencies considerably higher than resonance, the output signal decreases at precisely -40 dB/decade. Phase lag between the drive signal and sensor output increases to 180 degrees as the frequency is increased beyond resonance.
  • This method makes use of the relationship between the drive voltage and pressure as shown in equations (15) and (16) where for constant temperature and a fixed gas species the pressure is proportional to the drive voltage for a fixed amplitude.
  • Fig. 6 shows a schematic for the pressure sensor interface electronics utilizing this method which is preferred, where the drive electrodes 62a and 62b are driven with phase splitter 61.
  • the phase splitter provides in-phase signals and phase shifted signals 180 degree out of phase to the drive electrodes.
  • a DC bias voltage is provided by the phase splitter sub circuit (not shown).
  • the mass 64 is suspended by torsion spring 63, which allows motion of the mass in a seesaw mode about the torsion spring suspension.
  • the mass is electrically connected to charge amplifier 65, which provides a virtual ground for the mass. Excitation from the drive electrodes causes the mass to vibrate around the torsion spring at its resonant frequency. This motion causes the sensor capacitance to vary in phase with the motion of the mass.
  • Bias voltage applied between the mass and drive electrodes is constant, and causes charge to flow in and out of the charge amplifiers virtual ground as the mass vibrates. The oscillating virtual ground current results in a voltage signal at the output of the charge amplifier.
  • the 400X voltage amplifier 66 provides additional gain for the pressure sensor signal.
  • Fig.7 is a schematic for the readout and drive electronics where the readout from the sensor is amplified 71 to a 10 volt z-p signal level.
  • This amplified signal is applied to analog multiplier (X 2 /10) 72 and low pass filter 73.
  • Output from the low pass filter is a voltage that is 1/2 the signal amplitude which is applied to the squaring circuit (X 2 /10).
  • the low pass output signal and a 5 volt set point voltage are applied to integrator 74. Any difference between the set point and the low pass filter is integrated thereby producing a control signal.
  • the amplified 10 volt z-p sensor signal is also applied to an additional integrator 75 designed to shift the phase +90 degrees.
  • This signal (+90 degree phase lead) now has the correct phase to excite the sensor.
  • the control signal and the +90 degree phase shifted signal are applied to an additional multiplier 76 that modulates the amplitude of the +90 degree shifted signal.
  • the constant amplitude method is preferred for the simplicity of the electronics and the readout speed.
  • All the electronic drive and readout methods described can be used for squeeze damped and molecular damped sensors and can be extended to the slip damped regime by recognizing the change in the dependence of the quality factor on pressure in this regime.

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Claims (20)

  1. Mikromechanisches Druckmessgerät zur Verwendung mit einem Gas mittlerer freier molekularer Weglänge, aufweisend:
    ein stationäres Element, wobei das stationäre Element aus einem elektrisch isolierenden Material hergestellt ist, und zumindest eine Substratelektrode aufweist, wobei jede Elektrode einen jeweiligen Substratleiterpfad aufweist, der mit ihr übereinstimmt und der mit ihr betriebsmäßig verbunden ist,
    mehrere Vibrationselemente, die an dem stationären Element benachbart zu diesem fest angebracht sind, wobei die Vibrationselemente jeweils eine Masse aufweisen, eine Einrichtung zum Tragen der Masse, eine Masseelektrode, die mit der Masse übereinstimmt und betriebsmäßig mit dieser verbunden ist, und einen Masseleitungspfad, der betriebsmäßig mit der Masseelektrode verbunden ist und der elektrisch die Vibrationselemente mit den stationären Element verbindet, um im wesentlichen unter einer Distanz zu liegen zu kommen, die mit der mittleren freien Weglänge von dort aus vergleichbar ist,
    einen elektrischen Oszillator, der mit den Leitungspfaden verbunden ist, um mechanische Vibrationen einer vorbestimmten Oszillationsamplitude in jedem Vibrationselement bei Vorliegen einer druckabhängigen Dämpfung einzuleiten und aufrecht zu erhalten, so dass die Vibrationselemente jeweils über einen festgelegten Abschnitt eines ausgedehnten molekularen Mitnahmebereichs arbeiten, und
    eine Einrichtung zum Messen einer Treiberspannung, die zum Aufrechterhalten der vorbestimmten Masseoszillationsamplitude benötigt wird, wobei die Treiberspannung proportional zu dem Gesamtdruckwert des Gases ist.
  2. Messgerät nach Anspruch 1, wobei das Vibrationselement bzw. die Vibrationselemente (41, 51) aus Silizium besteht bzw. bestehen, und wobei die Einrichtung zum Tragen des Vibrationselements bzw. der Vibrationselemente aus zumindest einem Paar von Federn (42a-d, 55a-d) besteht bzw. bestehen, wobei die Federn (42a-d, 55a-d) einen Q-Wert größer als 100.000 aufweisen.
  3. Messgerät nach Anspruch 2, wobei es sich bei den Federn (42a-d, 55a-d) um Torsionsfedern handelt, und wobei das Vibrationselement bzw. die Vibrationselemente (31, 64) Massen aufweist bzw. aufweisen, die einander diametral gegenüberliegen und an dem stationären Element (33) in Sägezahnkonfiguration fest angebracht sind.
  4. Messgerät nach einem der Ansprüche 1 bis 3, wobei es sich bei dem Vibrationselement bzw. den Vibrationselementen (21, 31, 41, 51, 64) um Platten vielseitiger Form handelt.
  5. Messgerät nach einem der Ansprüche 1 bis 4, wobei das Vibrationselement (21, 31, 41, 51, 64) in mehreren unterschiedlichen Vibrationsmodi oszilliert, oder wobei jedes Vibrationselement in einem anderen Vibrationsmodus vibriert.
  6. Messgerät nach Anspruch 5, wobei die Vibrationsmodi Oszillationsrichtungen parallel sowie senkrecht zu dem stationären Element (22, 33, 47, 52) sind.
  7. Messgerät nach einem der Ansprüche 1 bis 6, wobei die Druckmessung auf höhere Drücke erweitert ist durch Vorsehen von mehreren Öffnungen (48) in dem Vibrationselement (41), damit das Gas dort hindurch gelangen kann, wobei die Öffnungen (48) voneinander in der Größenordnung von etwa einer Distanz beabstandet sind, die kleiner ist als ein Freiraumabstand zwischen dem Vibrationselement (41) und dem stationären Element (47).
  8. Messgerät nach Anspruch 2 oder einem von diesem abhängigen Anspruch, wobei die Federn (42a-d, 52a-d) das Vibrationselement (41, 51) mit dem stationären Element (47, 52) betriebsmäßig verbinden, um Oszillationen in einer Richtung senkrecht zum stationären Element (47, 52) zu fördern.
  9. Messgerät nach Anspruch 2 oder einem von diesem abhängigen Anspruch, wobei die Federn (42a-d, 52a-d) das Vibrationselement (41, 51) mit dem stationären Element (47, 52) betriebsmäßig verbinden, um Oszillationen in einer Richtung parallel zu dem stationären Element (47, 52) zu erleichtern.
  10. Messgerät nach Anspruch 9, wobei die stationären Elementelektroden (45, 53a, 53b, 54) mit den Vibrationselementelektroden fluchtend angeordnet sind, um Oszillationen parallel zu dem stationären Element zu erzeugen.
  11. Messgerät nach einem der Ansprüche 1 bis 10, wobei das Vibrationselement (21, 31, 41, 51, 64) dazu ausgelegt ist, in einer Ebene zu oszillieren, die parallel zu der ebenen Oberfläche des stationären Elements (22, 33, 47, 52) angeordnet ist, wobei die natürliche Oszillationsfrequenz im Bereich von 20.000 Hz bis 100.000 Hz liegt.
  12. Messgerät nach einem der Ansprüche 1 bis 11, wobei das Vibrationselement (21, 31, 41, 51, 64) außerdem ein Masse-Federsystem umfasst, wobei das System ein elektrisch leitendes Material aufweist.
  13. Messgerät nach Anspruch 12, wobei das elektrisch leitende Material ein kristallines Silizium ist, das mit Bor dotiert ist.
  14. Messgerät nach einem der Ansprüche 1 bis 13, wobei das Vibrationselement (21, 31, 41, 51, 64) dazu ausgelegt ist, entlang einer direktionellen Achse senkrecht zur ebenen Oberfläche zu oszillieren, wobei die natürliche Oszillationsfrequenz im Bereich von 20.000 Hz bis 100.000 Hz liegt.
  15. Messgerät nach einem der vorangehenden Ansprüche, wobei die Distanz zwischen dem stationären Element und dem Vibrationselement zwischen 1 und 1/100 einer mittleren freien Weglänge beträgt, und wobei der gemessene Druckbereich dadurch über den molekularen Mitnahmebereich in einen Gleitmitnahmebereich erweitert ist durch Kompensieren von Änderungen der Treiberspannung und der Oszillationsamplitude, resultierend aus Betriebsabläufen in dem Bereich.
  16. Messgerät nach einem der Ansprüche 1 bis 15, wobei das stationäre Element (22, 33, 47, 52) aus elektrisch isolierendem Material hergestellt ist.
  17. Verfahren zum Erweitern eines Messbereichs für ein mikromechanisches Druckmessgerät, aufweisend die Schritte:
    Bereitstellen von zumindest zwei Vibrationselementen mit einer Oszillationseinrichtung, die aus einem Material mit einem hohen Q-Wert hergestellt und einem stationären Element mit einem minimalen Freiraum zwischen den Vibrationselementen und dem stationären Element festgelegt ist,
    Erweitern des Messbereichs auf höhere Drücke in sowohl einem molekularen Mitnahme- wie einem Gleitmitnahmebereich durch Treiben eines ersten Vibrationselements zugunsten einer Oszillation in einer Richtung parallel zu dem stationären Element und Minimieren der Resonanzfrequenz des Vibrationselements,
    Erweitern des Messbereichs auf niedrigere Drücke durch Treiben eines zweiten Vibrationselements zur Oszillation in einer Richtung senkrecht zu dem stationären Element und Maximieren einer minimalen ebenen Abmessung des Vibrationselements und Minimieren der natürlichen Frequenz des Elements.
  18. Verfahren zum Erweitern eines Messbereichs für ein mikromechanisches Druckmessgerät, aufweisend die Schritte:
    Bereitstellen von zumindest zwei Vibrationselementen mit einer Oszillationseinrichtung, die aus einem Material mit einem hohen Q-Wert hergestellt ist, und das an einem stationären Element festgelegt ist, wobei ein Freiraum zwischen den Vibrationselementen und dem stationären Element kleiner als eine mittlere freie Weglänge von Molekülen eines Gases zwischen dem Vibrationselement und dem stationären Element bei einem höchsten gemessenen Druck ist, wobei der höchste Druck innerhalb eines molekularen Mitnahmebereichs liegt,
    Erweitern des Messbereichs auf höhere Drücke durch Treiben eines ersten Vibrationselements, das eine perforierte Oberfläche aufweist, zugunsten einer Oszillation in einer Richtung senkrecht zu dem stationären Element und
    Minimieren der Resonanzfrequenz des Vibrationselements, Erweitern des Messbereichs auf niedrigere Drücke durch Treiben eines zweiten Vibrationselements zugunsten einer Oszillation in einer Richtung senkrecht zu dem stationären Element, Minimieren des Freiraumabstands zwischen dem Vibrationselements und dem Tragelement, Maximieren einer minimalen ebenen Abmessung des Vibrationselements und Maximieren der Resonanzfrequenz des Vibrationselements.
  19. Verfahren zum Erweitern des Messbereichs nach Anspruch 17 oder 18, wobei die Niederdruckmessung in einem Bereich von 1,33 x 10-4 Pa bis 1,33 Pa liegt, und wobei der Hochdruck in einem Bereich von 0,133 Pa bis 1,33 x 105 Pa liegt.
  20. Verfahren zum Erweitern eines Messbereichs für ein mikromechanisches Druckmessgerät, aufweisend die Schritte:
    Bereitstellen eines Vibrationselements (21, 31, 41, 51, 64) mit ebenen Abmessungen, die eine minimale Abmessung für eine gegebene Elementform maximieren, wobei das Element an einem stationären Element (22, 33, 47, 52) mit minimierten Freiraum festgelegt ist, wobei das Vibrationselements (21, 31, 41, 51, 64) ein Federsystem mit sowohl niedriger wie hoher natürlicher Frequenz ist, die Oszillationen des Vibrationselements (21, 31, 41, 51, 64) sowohl in parallelen wie senkrechten Richtungen relativ zum stationären Element (22, 33, 47, 52) zulässt, Oszillierenlassen des Vibrationselements (21, 31, 41, 51, 64) in einer Ebene parallel zu dem stationären Element (22, 33, 47, 52) für Hochdruckmessungen, und
    Oszillierenlassen des Vibrationselements (21, 31, 41, 51, 64) in einer Ebene senkrecht zu dem stationären Element (22, 33, 47, 52) für Niederdruckmessungen.
EP96301794A 1995-03-21 1996-03-15 Mikromechanischer Drucksensor mit erweitertem Messbereich Expired - Lifetime EP0735354B1 (de)

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US407840 1989-09-15
US08/407,840 US5528939A (en) 1995-03-21 1995-03-21 Micromechanical pressure gauge having extended sensor range

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US5528939A (en) 1996-06-25
EP0735354A1 (de) 1996-10-02

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